We often encounter interefence problems when three phase motor variable speed drives are installed together with PLC and DDC control equipment.

Avoiding and/or correcting this problem seems to be more of an art than a science. Lots of trial and error and often expensive overkill.

Should PLCs be installed in the same panel with VSDs? Do multiple VSDs ineract? What's good practice for power, low voltage, communications cabling? Do filters help?

We'd like to hear about your field experience with these problems.

 

I have routinely seen VFDs and VSDs which are intstalled in a enclosure with a PLC to cause all kinds of problems, even effecting radio communications. Filters are a good idea, always seperate low voltage, signal and communications wires and cables from any high voltage, keep enclosures closed, attach ground bonding wires to the doors and get someone to come out with a spectrum analyzer and tell you where the noise is from. You can also rent or purchase a power line analyzer to determine the noise being generated by the VSD - VFD devices. Talk with the VSD manufacture, I am sure they have lots of technical experience in this area and can recommend the appropriate filters. It is not black magic, you just need the right instruments to look at what is causing the problem. Good grounding, good wiring pratices, filters as required and keeping enclosures closed are good places to start and usually make a world of difference.

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Technical Consutlants
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Hello Zvi

The main thing to realize about VSDs is that they generate high frequency harmonics in the output to the motor. You must use screened cable between the VSD and the motor or else this cable will radiate the harmonics to all electronic equipment in the vicinity. Cable manufacturers make cables specifically for this purpose, with the copper screen also used as the earth conductor. You should also route these cables well away from instrumentation cables.

For very large VSDs (over 150kW) where single core cables must be run to the motor, you can't use screened cable. In this case, route the cables along a covered metal cable way with no other cables.

I would not recommend installing a VSDs in the same panel as PLCs or other instrumentation.

Regards
Jamie Downs
New Zealand

 

1. Decrease pulse frequency of the VFD.
2. Place the VFD in a separate part of the cubicle (Faraday cage)
3. Use EMC filter
4. Use screened cable for power and signal lines.
5. Do not lay power cables in parallel with signal cables. Power cables should only cross signal cables at 90 degrees.

 

We have installed PLC controlled VSDs in the 500-1000 HP range (for plastic extruders). We have not encountered any interference problems to date.

Some advice:

NEVER run low-voltage, including communication wiring in the same conduit or cable tray with power voltage. You should maintain minimum 1 meter distance between low- and high-voltage wiring. If low voltage wiring must cross high-voltage wiring, do so perpendicularly (at right angles.) Use conduit or shielded cable tray.

DON'T install PLCs in the same cabinets as VSDs unless the vendor recommends it and is willing to take the financial hit when it doesn't work.

ALWAYS follow the vendor's advise. It's their product and should be included in implementation/installation issues.

Multiple VSDs may interact through regenerative noise on a shared power buss. Generally not an issue if you follow the vendor's recommendations.

In general, if vendor has a serial communication interface, such as DeviceNet or ProfiBus, we try to use that whenever possible. Saves quite a bit on wiring costs, and usually provides much better access to drive features.

 

Here are a few ideas from my experience

1) Follow the manufacturer's recommendations for cabling, try to use cage clamps for the cable screens, some manufacturers can supply 'emc' connection kits, these are great. (Siemens, Telemecanique)

2) In Europe you are obliged by regulations to use filters in certain circumstances, for instance with long motor cables.

3) Consider remote mounting of the inverter (Micromaster, Movimot, Danfoss, Lenze) using a fieldbus for control (USS, Profibus, CAN)

4) Be scrupulous with your earthing (grounding), use the recommended earthing cable size and ensure that cable screens are drained.

5) Check the switching frequency of your inverters, higher switching frequencies can cause interference, especially in systems with common low voltage supplies (many of the systems I integrate with have a common +24v supplied, if this line is poorly decoupled it can transmit problems)

marc sinclair

 

> I would not recommend installing a VSDs in the same panel as PLCs or other instrumentation. <

This is paranoia, for over fifteen years I have used many different PLC and inverter combinations always on the same panel, always within 50cm and this has never caused a problem. Good design layout is essential and read the manufacturers' instructions

Marc Sinclair
http://www.germainesystems.co.uk